Communication systems that utilize coded communication signals are well known in the art. One such system is a code division multiple access (CDMA) cellular communication system such as set forth in the Telecommunications Industry Association/Electronic Industries Association International Standard (TIA/EIA IS-95), hereinafter referred to as 15-95. In accordance with the IS-95, the coded communication signals used in CDMA systems comprise CDMA signals' that are transmitted in a common 1.25 MHz bandwidth to base stations of the system from mobile or wireless communication units, such as cell phones, portable wireless computers, or wireless handheld devices, that are communicating in a specific coverage area of the base station. In conventional CDMA systems, the base station communicates with abase station controller which allows the communication unit to communicate with other communication units within the same coverage area. Each CDMA signal includes a pseudo-noise (PN) sequence associated with a particular base station and an identification number of a communicating communication unit.
Typically, the base station controller is connected to a mobile switching controller (MSC) which allows a base station to connect with other base stations outside its coverage area in order to allow a communicating communication unit to communicate with other units outside its coverage area.
FIG. 1 illustrates a conventional CDMA communication system 100 including a first base station 110, a second base station 120, and one or more communication units 105, 106. The communication system 100 illustrated in FIG. 1 is an exemplary CDMA system which includes a direct sequence CDMA cellular communication system, such as that set forth in TIA/EIA IS-95.
In the system shown in FIG. 1, the base stations 110 and 120 are connected to a base station controller (BSC) 130 and a mobile switching controller 140 which is in turn connected to the public switched telephone network (PS TN) 150 using known techniques.
The system shown in FIG. 1 further connects to the public land mobile network (PLMN) to allow mobile communication units to travel from one network to another roaming while maintaining a subscriber profile information. A detailed illustration of the PLMN is shown in FIG. 1B. In the system shown in FIG. 1B, a conventional cellular (or PCS) wireless communication network is shown. In the network shown in FIG. 1B, a network subscriber's profile information is typically stored and maintained in a home location register (HLR). One HLR is typically required per service provider.
Typically when a new subscriber is activated, the service profile such as the subscriber's charging rate and service restrictions (e.g. no international call allowed) are included in the profile. The service profile of all communication units within a network is stored in the HLR located in the home network of the communication unit. Since a user in one cellular (or PCS) network can move into another network and use services within that network, communication exists between networks to share service profiles. For example, if a subscriber subscribing services from a service provider in a particular state (e.g. GTE mobile services in State 1) travels to another state with a different service provider (e.g. CellularOne in State 2) and attempts to make a call in State 2, the cellular system in State 2 will query the HLR in State 1 for the service profile of the network user.
This type of inter-networking communication is carried out using ANSI-41D mobile application protocol. The conventional cellular network uses ANSI-41D protocol on top of SS7 transport protocol to handle all inter-networking communications. These inter-networking communications are very costly and inflexible. The SS7 interface card, which enables the inter-networking communication, can cost upwards of $10,000. The SS7 interface card also has some configuration inflexibility due to the proprietary nature of the interface cards.
Still referring to FIG. 1, when a communication unit initiates a call sequence to either one of the base stations 110 or 120 within a coverage area, an end-to-end connection is established between the respective base station, the base station controller 130, and the MSC 140 using known CDMA call setup techniques. The base stations 110 and 120 typically communicates with the BSC 130 and the MSC 140 via communication links, such as a T1 connection. Base stations 110 and 120 typically have antennas to define the coverage area within which either base stations primary accommodate the communication units.
In the system shown in FIG. 1, when a communicating communication unit initiates a call sequence (uplink) to the nearest base station, the call is assigned to the target communication unit via the BSC 130 and the MSC 140 within a prescribed bandwidth (e.g. 1.25 MHz for 15-95).
In conventional CDMA systems, voice quality is degraded as more subscribers originate calls over the system, and as a communicating communication unit strays further away from its base station. Although, voice degradation can sometimes be acceptable to a user using the communicating communication unit because the user can always repeat an earlier transmitted message, data communication is more susceptible to degradation. This is because, in the case of data degradation, the sender of the data does not know at what point that data being sent by a communicating communication unit begins to degrade or is lost. Consequently, if data transmission begins to degrade or is lost, the communication unit will have to resend the entire data. Such retransmission can be costly.
Also, in the conventional CDMA system shown in FIG. 1, communication between a communicating communication unit and the base station requires a dedicated end-to-end connection between the base station, the base station controller, and the mobile switching controller. Such dedicated end-to-end connection can also be very expensive and time consuming.
Another problem with the conventional system described in FIG. 1 is that the communication interface between the base station and the base station controller requires proprietary interface technology which makes scaling the system to other communication platforms very cumbersome and expensive.
Furthermore, most of these conventional CDMA communication systems utilize a T1 or E1 communication pathway which have bandwidths of 1.544 Mbps for T1 connections and 2.04 Mbps for E1 connections, and are not known to handle data bursts and therefore are very slow for the transmission of data.
To alleviate some of the problems of the prior art, some prior art CDMA systems such as illustrated in FIG. 2 use multicarrier base stations to handle multiple calls and handoffs to and from the base stations. In this system, a communicating communication unit is able to utilize different carriers in the base station for transmitting and receiving calls.
Although such multicarrier systems may alleviate the problems with voice quality degradation, they do not handle data transmission very well. Thus, these prior art solutions do still have problems with the quality of data calls transmitted with a coverage area from the base stations.
In the exemplary CDMA system shown in FIG. 1, each base station transmits a pilot signal having a common PN spreading code that is offset in code phase from the pilot signal of other base stations within the system. During system operation, the mobile communication unit is provided a list of code phase offsets corresponding to neighboring base stations surrounding the base station through which communication is established. The mobile unit is equipped with a searching function which allows the mobile unit to track the signal strength of the pilot signal from a group of base stations including the neighboring base stations.
Various methods exist for switching the mobile communication unit from one base station to another (typically known as ‘handoff’). One such method is termed a “soft” handoff, in which communication between the mobile unit and the end user is uninterrupted by the eventual handoff from an original base station to a subsequent base station. In other words, communication with the subsequent base station is established before terminating communication with the original base station. When the mobile unit is communicating with two base stations, a single signal for the end user is created from the signals from each base station by a communication system controller.
Mobile unit assisted soft hand off operates based on the pilot signal strength of several sets of base stations as measured by the communication unit. An Active Set is the set of base stations through which active communication is established. A Neighbor Set is a set of base stations surrounding an active base station comprising base stations that have a high probability of having a pilot signal strength of sufficient level to establish communication.
When communications are initially established, the communication unit communicates through a first base station, and the unit monitors the pilot signal strength of the base station in the Active Set and the Neighbor Set. When a pilot signal of a base station in the Neighbor Set exceeds a predetermined threshold level, the base station is added to the Candidate Set and removed from the Neighbor Set at the communication unit.
The communication unit communicates a message identifying the new base station. The base station controller decides whether to establish communication between a new base station and the communication unit. Should the base station controller decide to do so, the base station controller sends a message to the new base station with Identifying Information about the communication unit and a command to establish communications.
When the communication unit is communicating with multiple base stations, it continues to monitor the signal strength of base stations to determine which base station to connect to in the event of a signal strength degradation.
Each base station has a coverage area that has two handoff boundaries. A hand off boundary is defined as the physical location between two base stations where the link would perform the same regardless of whether the mobile unit were communicating with the first or second base station. Each base station has a forward link hand off boundary and a reverse link hand off boundary.
The forward link “handoff boundary is defined as the location where the mobile unit's receiver would perform the same regardless of which base station it was receiving. The reverse link handoff boundary is defined as the location of the mobile unit where two base station receivers would perform the same with respect to that mobile unit. Ideally these boundaries should be balanced, meaning that they have the same physical location with respect to the base station. If they are not balanced, system capacity may be reduced as the power control process is disturbed or the hand off region unreasonably expands.
In any of these conventional systems, the soft handoff between base stations still require the active base station to maintain contact with the base station controller as it hands off communication to a neighboring base station or a candidate base station. Upon handing over communication, the new base station (now active base station) resumes communication with the mobile unit via the base station controller. The conventional system described in FIG. 1 or FIG. 2 does not allow each base station to communicate with the other during a handoff since all communication has to go through the base station controller. This takes time, and in a data traffic transmission it can be costly.
Therefore, it is desirable to have a system and a method for transmitting CDMA calls including voice and data over a communication pathway with a higher bandwidth. It is further desirable to have a CDMA system that handles the transmission of calls, especially data calls, without the inherent call quality degradation. A need further exists for an improved and less costly system which improves efficiency and the transmission rate and time of calls between a mobile unit and a base station, between base stations and a base station controller, and between adjacent base stations.